Hydrogen concentrations in methane-forming cells probed by the ratios of reduced and oxidized coenzyme F420

Abstract

Coenzyme F420is the central low-redox-potential electron carrier in methanogenic metabolism. The coenzyme is reduced under hydrogen by the action of F420-dependent hydrogenase. The standard free-energy change at pH 7 of F420reduction was determined to be −15 kJ mol−1, irrespective of the temperature (25–65 °C). Experiments performed with methane-forming cell suspensions ofMethanothermobacter thermautotrophicusincubated under various conditions demonstrated that the ratios of reduced and oxidized F420were in thermodynamic equilibrium with the gas-phase hydrogen partial pressures. During growth in a fed-batch fermenter, ratios changed in connection with the decrease in dissolved hydrogen. For most of the time, the changes were as expected for thermodynamic equilibrium between the oxidation state of F420inside the cells and extracellular hydrogen. Also, methanol-metabolizing, but not acetate-converting, cells ofMethanosarcina barkerimaintained the ratios of reduced and oxidized coenzyme F420in thermodynamic equilibrium with external hydrogen. The results of the study demonstrate that F420is a useful probe to assessin situhydrogen concentrations in H2-metabolizing methanogens.